Too much compression??

Is it possible to have too much compression? I just did a compression test on my bike. I got 150 to 152 psi on all four. Did the test with wide open throttle. Then checked the factory service manual and it states that standard is 121 psi, minimum is 100 psi and Maximum is 135 psi At sea level. The only thing I did different was not warm up the engine. Did not think this would make defiance.

 

3 Possibilities:

1 - Spec is wrong (Clymer also says 121).
2 - Gauge is reading incorrectly
3 - Lots of carbon on the pistons/heads

My bet is the gauge is bad.

 

The gauge is new and of good qualty it is new is on loan from my brother a auto repair tec. The bike runs great. What numbers are other 550 owners getting?

 

Mine produce 130~132 across the board; engine slightly warm.

 

Are you the original owner, it's amazing what people do to bikes and then sell them on.
Shaved head, after market pistons etc.

 

Yea I was gonna add that 4th possibility. If they had the head milled it could do that, easy to gain 10-15% overall psi with a smaller combustion chamber.

Spec isn't wrong that's what it is from Yamaha. My 550 pumps 125-132.

Does it burn any oil?

 

I'm not the original owner but I don't think the head has been milled. the bike had a rough life before I saved her. It is not burning any oil. The plugs are tan in color.

 

Ok now I'm really curious. What kind of oil are you running?

Do you're numbers match from your frame and engine?? And are your last 6 digits a very small number like 000075?

 

What's the last digits being small have to do with anything?

Just curious.

 

Possible production changes. Japanese manufacturers have been known to alter specs after an initial short production run and management makes changes due to testing. In the cases I know about they have altered gear ratios, cam profiles and compression. Digging for clues....

 

Interesting! Mine's #751, which while i thought was somewhat low, i don't think would be low enough to be affected by something of the like.

I dont have any issues, i was moreso just curious, lol.

thanks for that neat bit of info!

 

Nice theory, except the XJ550 had been around for a few years first as a 400 then a 500 before we got them as the 550s.

I'm betting wacky gauge. I'd recommend re-testing with a different gauge.

 

. . . or test the gauge on a car and see if it reads 200 or so.